Hmmm. I think, and believe me I could be wrong, that it is 21 days. Here's why: Carol 2 x 24 = 48
Gideon 2 x 24 = 48
Jose 2 x 24 = 48
Mary 1 x 24 = 24
____
168
There are 168 cookies

Now Mary eats an extra 1 a day
So they are eating 8 cookies a day
168 divided by 8 is 21. So, I think it is 21 days.

Or I lose and I have to say to the world, I am not smarter than 4th grader.